WordTips is your source for cost-effective Microsoft Word training. (Microsoft Word is the most popular word processing software in the world.) This tip (92) applies to Microsoft Word 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Word (Word 2007 and later) here: Sequentially Numbering Elements in Your Document.

An invoice number is simply a way to uniquely mark an invoice to help keep track of your business transactions. Consecutive or sequential invoice numbers help to ensure that each invoice is unique and each business transaction can be clearly and comprehensively organized and referenced – for accounting reasons as well as customer support. While a receipt is generally used as proof of payment and therefore documents a finalized sale, an invoice is a request for payment that includes more detailed payment or contact information in addition to pricing information for the goods or services rendered. Having a proper invoice numbering system helps you avoid duplicate payments that can amount to large losses for your company.
Sequential numbers can be printed almost anywhere on a sheet or form and can be positioned horizontally or vertically. Numbers can also be repeated in another position on the form. When developing your artwork, consider putting a box for numbering, making it easier for your customers or employees to find and reference a specific job or transaction. You may also differentiate your number by color. Most printers allow a choice of colors, typically black or red, to make your number stand out. Start your sequencing at any point you like, to pick up where you left off on your last print order.